MIC4950 Datasheet, PDF (11/18 Pages) Micrel Semiconductor – Hyper Speed Control 5A Buck Regulator

Micrel, Inc.
When the required duty cycle is very low, the required
OFF time is typically far from the Minimum OFF Time limit
(about 176 ns typ). In this case, the MIC4950 operates by
delivering at each switching cycle a determined ON time
(dependent on the input voltage). A new ON time is
invoked by the error comparator when the FB voltage
falls below the regulation threshold. In this mode the
MIC4950 operates as an adaptive Constant-ON-Time
ripple controller, with nearly constant switching
frequency. Regulation takes place by controlling the
valley of the FB ripple waveform.
When higher duty cycles are required, regulation can no
longer be maintained by decreasing the OFF time below
the Minimum OFF Time limit. When this limit is reached,
then the OFF Time is no longer reduced, and the
MIC4950 smoothly transitions to an ON-time modulation
mode. In the ON-time modulation region, frequency
reduces with the increase of the required ON-time/duty
cycle, and regulation finally takes place on the peak of
the FB ripple waveform.
Note that because of the shift of the regulation threshold
between different modes, line regulation might suffer
when the input voltage and/or duty cycle variations force
the MIC4950 to switch form one regulation mode to the
other. In applications where wide input voltage variations
are expected, ensure that the line regulation is adequate
for the intended application.
MIC4950
March 20, 2014
11
Revision 1.1